Pressure-actuated metered dose inhaler (pMDI) devices are designed to dispense a predetermined dose of a medicament to a user or patient when actuated. The medicament is stored and dispensed either as a fluid of active particles in a suspension or with the active agent dissolved in the fluid. The operation of a valve within the device is used to dispense the dose of medicament from a metering region once the device is activated. However, dosing variation, that is, dose enrichment or dose depletion, may be experienced when particles in the suspension migrate in the direction of decreasing fluid turbulence (which can be into or out of the metering chamber), resulting in variations in the actual dose of the medicament dispensed to the patient. This migration is a phenomenon known as turbophoresis.
WO-A-02/10037 describes ways of closing off a valve metering chamber when the pMDI device is at rest in order to prevent dose enrichment or dose depletion effects directly into or out of the metering chamber. However, dose enrichment or dose depletion effects occurring in a critical pre-metering region of the valve are not addressed so that, even when the metering chamber is closed off at rest, dose enrichment or dose depletion can still occur due to particles migrating into or out of the pre-metering region. This means that a patient may not always take the correct dose of his/her medicament due to no fault of his/her own.